Let's look at this as simply as possible:
1) The last time Chris stopped sending letters to the outside world for a significant period of time was when he was in the psychiatric hospital, which did not allow him to send and receive letters.
2) Chris' Lawyer is trying to use Chris' mental health and autism as a defense in the trial. That requires an analysis by medical experts, requiring him to be sent back into a psychiatric hospital.
Therefore, the letters have stopped because Chris has been returned to a psychiatric hospital for an analysis pending the autism defense.
Also, regarding Barb's weight, I do think Bob's death was partly responsible, but not because Barb felt any grief. Rather, without an authoritatively-voiced man willing to yell at him, Chris no longer felt as much need to do what he was told by Barb and run and complete her errands on the fly. As she aged, she became less able to go and get her own food, and the power relation between the two of them shifted to the point where Chris no longer feared her or felt he had to obey her (Barbussy proves this). So, she was stuck in the house in which she was unable to cook anything due to her own hoarding, and Chris probably just fed her whenever he could be bothered to get an extra Q-Sand to bring home from his trips to McDonalds. The rest of the time he just tossed her an Ensure and called it a day.